(1) Field of the Invention
This invention relates to an unwoven metal fabric which is a kind of porous metal structure for various uses such as battery electrodes, catalysts and filters and a method of manufacturing the same.
(2) Description of the Prior Art
Conventional battery electrodes and filters are formed from ultra-fine metal fibers entangled together. Different materials and various new techniques for forming them have been developed.
For example, Unexamined Japanese Patent Publication 4-11058 discloses a material made of metal fibers that are joined together so that the length of the joint portions will be more than 0.7 time the average fiber diameter, and a method of manufacturing the same. Unexamined Japanese Patent Publication 3-17957 proposes a material made of metal fibers which are subjected to pre-plating treatment using precious metal catalysts to improve the adhesion of plating to the fibers, and a method of manufacturing the same.
Also, Unexamined Japanese Patent Publication 61-76686 proposes to provide a metal coating on a felt member by sputtering under vacuum in order to provide an electrode having a high porosity. But the methods disclosed in these publications require complicated processes such as preplating treatment by catalysts and sputtering, so that mass-productivity is low. Also, if the electrodes thus formed contain more than 100 ppm metallic impurities resulting from the catalysts used, their properties will vary so widely that they will not stand practical use.
In order to solve these problems, Unexamined Japanese Patent Publication 4-126859 proposes a metal fiber material formed by electroplating a high-conductivity carbon fiber sheet and removing the substrate. The carbon fiber sheet used here is manufactured by forming a mixture of carbon fibers and a substance for bonding fibers (hereinafter referred to as "binder") into sheet form and drying it.
The carbon fibers in such a carbon fiber sheet have a resistivity of mere 10.sup.-3 -10.sup.-4 .OMEGA..multidot.cm. But the binder surrounding the carbon fibers has a very high resistivity because it is an insulating material. It is not an easy job to directly electroplate such a sheet. It would be possible to reduce the content of the binder in order to increase the conductivity of the entire sheet. But this will make it necessary to increase the amount of plating to make up for the reduced adhesion between fibers. The metal fiber product thus obtained would have a porosity of 80-90%, which is too low for use as battery electrodes.
It is possible to reduce the resistivity of the fiber material by increasing its firing temperature to 1000.degree. C. or higher to graphitize the fiber. But since the fibers are joined together by the resin binder, it is impossible to electroplate such joint portions. The metallic porous member thus formed tends to be low in mechanical strength and unsatisfactory in its electrical properties. Also, excessive graphitization would make it necessary to increase the temperature to 900.degree. C. or higher in removing the substrate by thermal oxidation in the later step. This will in turn result in reduced strength of the unwoven fabric formed.
One solution to these problems is to increase the thickness of plating to 500-1000 g/m.sup.2 because such a thick plating layer provided on the carbon fiber portions overhangs the binder portions. But it is not easy to provide a thick plating having a uniform thickness over the entire area. Also, such a thick plating layer tends to keep a web form because it is difficult to roll. This solution is therefore not suitable for continuous production. Thus, it is an ordinary practice to increase the conductivity of the material by electroless plating and then provide an electroplating layer of a predetermined thickness.
The unwoven fabric formed by this conventional method shows high mechanical strength because plating is provided between fibers. But due to the properties of the plating solution, variations in the distribution of fibers tend to widen. The unwoven fabric thus formed is low in quality stability. If this unwoven fabric is used for batteries, the content of metallic impurities which have been used as catalysts will exceed 100 ppm, so that the variations in the properties of electrodes will increase to such a extent that such electrodes are practically useless. It is therefore desired to provide a method for manufacturing a stable metallic unwoven fabric without reducing the number of joint portions between metal fibers.